As an ion exchange membrane employed for an alkali chloride electrolytic process which comprises electrolyzing an aqueous alkali chloride solution such as seawater to produce an alkali hydroxide and chlorine, a membrane made of a fluorinated copolymer having carboxylic acid type functional groups or sulfonic acid type functional groups, has been known. The fluorinated copolymer is obtained by copolymerizing a fluorinated olefin with a fluorinated monomer having a carboxylic acid type functional group or a sulfonic acid type functional group (Patent Documents 1, 2 and 3).
In the alkali chloride electrolytic process employing an ion exchange membrane, it is important to suppress deterioration of the performance of the ion exchange membrane as far as possible so as to operate at a low cost. The deterioration of the performance of the ion exchange membrane is classified into a physical damage due to pinholes or deterioration of membrane, and a chemical damage due to e.g. a severe operating environment. A main component of the chemical damage may be deposition of impurities such as calcium or strontium in the ion exchange membrane.
Impurities in an aqueous alkali chloride solution are usually suppressed to be a low concentration by a purification step, but the quality of an alkali chloride to be used and a level of purification techniques vary depending on a user, and therefore such deposition of impurities in an ion exchange membrane frequently occurs among a lot of users, such being problematic.
If an ion exchange membrane is contaminated with impurities in an aqueous alkali chloride solution, the membrane resistance increases or current efficiency decreases. If the current efficiency decreases, there are considerable demerits such as an increase in running costs due to an increase of an electric power unit or an increase of the frequency of membrane exchange, and deterioration in quality of chlorine due to an increase of an oxygen concentration in chlorine. Accordingly, an ion exchange membrane with a small decrease in current efficiency by impurities in an aqueous alkali chloride solution, has been desired.
For example, when a drop width of a current efficiency is lowered by 1%, it is possible to suppress the increase of the electric power unit by 1% in production of an alkali hydroxide or chlorine, and it is thereby effective to reduce about 20,000,000 JPY for the cost of electricity in one year in the case of e.g. a plant to produce sodium hydroxide in an amount of 200,000 tons in one year. Further, in consideration of benefits such as a decrease in running cost due to extension of membrane life span or stabilization of electrolytic operation, it is crucially more important than suppression of an increase in electricity cost.
An ion exchange membrane is usually a laminate having a layer made of a fluorinated copolymer having sulfonic acid type functional groups, which can maintain strength, and a layer made of a fluorinated copolymer having carboxylic acid type functional groups, as a functional layer to develop high current efficiency. Since it has been known that the decrease in current efficiency by impurities occurs when the impurities are deposited on the layer made of a fluorinated copolymer having carboxylic acid type functional groups, it is essential to improve the layer made of a fluorinated copolymer having carboxylic acid type functional groups, for improving the current efficiency.